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Sunlight takes about eight minutes to reach you. The most distant galaxy Hubble has measured sent out the light we see more than 13 billion years ago. Between those two numbers lies a universe that is far emptier, older and stranger than everyday intuition suggests.
Cosmology is also a field where numbers come with honest margins of error, so a few figures below are labeled as estimates. Here are 5 facts about the cosmos that put everyday problems into perspective.
1. The universe is mostly empty space
If you added up every atom in everything that exists — stars, planets, cosmic dust — matter would still make up a tiny fraction of the observable universe. The rest is, essentially, vacuum. Measurements of the cosmic microwave background by ESA’s Planck satellite put ordinary matter at just 4.9 percent of the universe’s total mass and energy, with dark matter at 26.8 percent and dark energy at 68.3 percent.
Spread across all of space, even the total density is almost absurdly low: on the order of a few hydrogen atoms in every cubic meter, and well under one atom per cubic meter once you count only ordinary matter. Stars and planets are rare, dense exceptions in a very thin cosmic soup.
2. The starlight you see might be “from the past”
Since light takes time to travel, when you look at a distant star, you’re seeing it as it was years, centuries, or millennia ago — some of them might not even exist anymore. NASA points out that we see the Sun as it was about eight minutes ago, and the nearby Andromeda galaxy, roughly 2.5 million light-years away, as it looked about 2.5 million years ago.
The effect scales up dramatically with distance. The farthest galaxy Hubble had observed at the time NASA wrote its explainer, GN-z11, is seen as it appeared 13.4 billion years ago, within about 400 million years of the Big Bang. This is why astronomers call big telescopes time machines: looking farther away is looking further back.
3. The universe’s expansion is accelerating
Not only is the universe expanding, that expansion is speeding up over time — a phenomenon attributed to mysterious “dark energy,” something scientists still can’t fully explain. Two teams measuring distant Type Ia supernovae reached the same surprising result in the late 1990s: the explosions looked fainter than expected, as if the cosmos had been expanding more slowly long ago.
The discovery earned Saul Perlmutter, Brian Schmidt and Adam Riess the 2011 Nobel Prize in Physics. NASA describes dark energy as the energy of empty space and says it may make up about 70 percent of the universe, while stressing that its exact properties, and whether it matches Einstein’s cosmological constant, remain to be pinned down.
4. There are more stars in the universe than grains of sand on Earth
Estimates put the number of observable stars at around 10 followed by 22 zeros — a number that far exceeds the amount of sand on every beach and desert on the planet combined. ESA’s own rough range runs from 10^22 to 10^24 stars, built by multiplying roughly 10^11 to 10^12 stars per galaxy by 10^11 to 10^12 galaxies.
A back-of-envelope count of Earth’s sand, made by University of Hawaii researchers, lands near 7.5 x 10^18 grains. Even ESA’s low-end star figure is more than a thousand times larger. Both numbers are very rough, since they rest on average grain sizes and average galaxy sizes.
5. The universe will eventually “cool” to near absolute zero
One theory about the universe’s ultimate fate is “heat death”: as expansion continues, energy keeps dissipating until practically nothing happens anymore — a process that would take trillions of years. NASA’s fate-of-the-universe material describes one version of this picture, in which galaxies drift so far apart that they pass beyond the observable edge and the sky fades to an icy black.
It is only one scenario. NASA stresses that the outcome depends on the true nature of dark energy, and other possibilities, such as a “big rip” in which expansion eventually tears apart bound objects, remain on the table. For now, the timescales involved dwarf the 13.8 billion years the universe has existed so far.
Why the observable universe is 93 billion light-years wide
The universe is about 13.8 billion years old, so you might expect its visible portion to be 27.6 billion light-years across. The commonly quoted figure is about 93 billion. The difference is expansion: while light traveled toward us, the space it crossed kept stretching, so the sources of that ancient light now sit much farther away than the distance the light itself covered.
This also means “observable” is not the same as “everything.” It marks the region from which light has had time to reach us, and the whole universe may be far larger, possibly without limit.
The oldest light: a glow from 375,000 years after the beginning
NASA’s WMAP mission mapped the cosmic microwave background, which it calls the oldest light in the universe, released about 375,000 years after the beginning. Before then, the cosmos was a hot fog of charged particles that trapped light; once atoms formed, the light could travel freely.
That afterglow has since cooled to just a few degrees above absolute zero, and NASA’s COBE satellite found it matches the spectrum of an ideal glowing object at 2.725 kelvin. Its temperature barely varies across the sky, differing by only about one part in 100,000.
Other reads that fit this theme: facts about the moon and international space station.
Prefer something closer to home or more extreme? Try black hole facts, the biggest James Webb discoveries and facts about the Milky Way.
Want a piece of the cosmos at home?
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Between the eight minutes sunlight needs and the 13.4 billion years behind GN-z11, every look at the sky is really a look at an archive of very different ages.